This invention relates generally to meltblowing apparatus and processes for producing electrically charged meltblown fabrics.
Meltblown nonwoven fabrics display excellent properties for many uses, one of which is liquid and gas filtration. Important filtration parameters such as efficiency and fluid pressure drop can be improved by embedding a static electrical charge within the fabric. In addition, electrically charged nonwoven fabrics may display improved tactile hand. The present invention applies a persistent electrical charge to nonwoven meltblown fabrics.
Meltblown fabrics are generally formed by extruding a molten thermoplastic resin through a die which consists of a horizontal row of small diameter orifices. High velocity sheets of hot air exiting from air passages located just above and below the orifices converge at the die discharge. The convergent air streams induce an aerodynamic drag force upon the extruded polymer fibers as they exit the die. The drag rapidly draws or attenuates the polymer into extremely fine fibers forming a fiber-air stream. The degree of fiber attenuation or, in other words, the final fiber diameter has a significant effect on the final properties of the fabric. The fiber-air stream is directly blown onto a collector apparatus. Here the fibers are deposited forming a nonwoven fabric or web. Nonwoven webs are held together by a combination of fiber entanglement and/or fiber cohesive sticking while still in the semi-molten state. By using a suitable collector apparatus the entire process can be more or less continuous. The term "fiber" includes filaments since the extruded polymer can be deposited as discrete fibers or continuous filaments.
The microscopic diameters(average diameter of 0.5 to 10 microns generally) of the extruded fibers of the meltblown web are well suited to filtering finely divided particles out of a gaseous or liquid fluid. Experimental studies have shown that applying a persistent electrostatic charge to the fibers improves the filter quality. Webs carrying an electrical charge are often called electrets. Nonwoven fibrous electret filters have higher efficiencies, lower fluid pressure drop during filtration, and longer life than non-charged filters. U.S. patents which disclose nonwoven fibrous electrets include U.S. Pat. Nos. 4,215,682, 4,375,718, 4,588,537, 4,592,815, and 4,904,174.
A method for applying an electrostatic charge to the molten or hot fibers during the fabrication process is disclosed in U.S. Pat. No. 4,215,682. The electrostatic charging of the molten or hot fibers permits the charges to migrate into the fibers(since its electrical resistance is lower) and remain trapped upon cooling. This increases the charge life of the electret.
In the processes disclosed in U.S. Pat. Nos. 4,215,682 and 4,904,174, the charges are applied by establishing within a region near the die discharge a corona zone of free electrons and ionized air. The extruded polymer fibers and air stream pass through the dense electron and ionized air field and are charged thereby. The external charging of the fibers limits the proximity of the electron and ionized air field. Because of the spacing required in these devices, the extruded polymer fibers are generally in a semi-molten or solidified state when they pass through the electron or ionized air field.